Modern RFID tags are only a few millimeters in size and comprise a chip, antenna and in some cases a battery (active). Some forms of RFID tags (passive) have no battery, but actually take power from the electromagnetic beams of a reader, and then send data back to the source. These tags can theoretically last forever, since they only send data or require power when actually being pinged by a reader device. Almost all RFID tags can be inserted into almost anything and do not require line of sight back to a reader. Some tags are so tiny that they have been glued to the backs of ants to track their behavior.